Ch 7: Membrane Structure and Function

2016

Ch 7: MembraneFrom Topic 1.3Essential idea: The structure of biological membranes makes them fluid and dynamic.Nature of science:• Using models as representations of the real world—there are alternative models of membrane structure (1.11).• Falsification of theories with one theory being superseded by another—evidence falsified the Davson-Danielli model (1.9).Understandings:• Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.• Membrane proteins are diverse in terms of structure, position in the membrane and function.• Cholesterol is a component of animal cell membranes.Applications and skills:• Application: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.• Skill: Drawing of the fluid mosaic model.• Skill: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Daniellimodel.• Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.Guidance:• Amphipathic phospholipids have hydrophilic and hydrophobic properties.• Drawings of the fluid mosaic model of membrane structure can be two dimensional rather than three dimensional. Individual phospholipid molecules should be shown using the symbol of a circle with two parallel lines attached. A range of membrane proteins should be shown including glycoproteins.

From Topic 1.4Essential idea: Membranes control the composition of cells by active and passive transport.Nature of science: Experimental design—accurate quantitative measurement in osmosis experiments are essential (3.1).Understandings:• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.• The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.Applications and skills:• Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.• Application: Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.• Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions (Practical 2).Guidance:• Osmosis experiments are a useful opportunity to stress the need for accurate mass and volume measurements in scientific experiments.Utilization:• Kidney dialysis artificially mimics the function of the human kidney by using appropriate membranes and diffusion gradients.Aims:• Aim 8: Organ donation raises some interesting ethical issues, including the altruistic nature of organ donation and concerns about sale of human organs.• Aim 6: Dialysis tubing experiments can act as a model of membrane action. Experiments with potato, beetroot or single-celled algae can be used to investigate real membranes.

Ch 7: MembraneFrom Topic 6.1 (introduced in HL 1 but covered in HL 2)Understandings:• Different methods of membrane transport are required to absorb different nutrients.

From Topic 6.5Understandings:• Neurons pump sodium and potassium ions across their membranes to generate a resting potential. From Topic 9.1 Understandings:• Active uptake of mineral ions in the roots causes absorption of water by osmosis.

From Topic 9.2 Understandings:• High concentrations of solutes in the phloem at the source lead to water uptake by osmosis.• Active transport is used to load organic compounds into phloem sieve tubes at the source.

Biological Membranes• Essential idea: The structure of biological membranes makes them fluid and dynamic • Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.• Membrane proteins are diverse in terms of structure, position in the membrane and function. • Amphipathic phospholipids have hydrophilic and hydrophobic properties.

• Plasma membrane: a boundary that separates the living cell from it’s non-living surroundings; made of amphipathic phospholipids and proteins • @ 8 nm thick• Controls chemical traffic• Unique structure based on the different types of phospholipids and

proteins found in the PM• Selectively permeable: allows some substance to cross more easily

than others

Different Models of PM• Using models as representations of the real world—there are alternative models of membrane structure • Skill: Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.• Skill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model. Membrane proteins are diverse in terms of structure, position in the membrane and function Falsification of theories with one theory being superseded by another—evidence falsified the Davson-Danielli model.

• Davson-Danielli Model “Sandwich” Model: In 1935, Hugh Davson and James Danielli suggest that the plasma layer is made of two layers of phospholipids that are each surrounded by a layer of protein

Pro’s and Con’s of this model?

Different Models of PMSkill: Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model. Using models as representations of the real world—there are alternative models of membrane structure Essential idea: The structure of biological membranes makes them fluid and dynamic.

• Singer-Nicolson “Fluid Mosaic” Model: In 1972, S.J. Singer and G. Nicolson proposed that the proteins are dispersed and inserted in the phospholipid bilayer with their hydrophilic regions facing the water

- This model was supported by freeze-fracture: http://www.sciencephoto.com/media/530082/view

Fluidity of the Plasma Membrane

• The fluidity of PM comes from the movement of the phospholipids and the proteins. • Lipids and proteins can

drift laterally switching places, but it rare to switch between phospholipid layers.

• Essential idea: The structure of biological membranes makes them fluid and dynamic.• Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.• Membrane proteins are diverse in terms of structure, position in the membrane and function. • Amphipathic phospholipids have hydrophilic and hydrophobic properties• The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.

Fluidity of the Plasma Membrane

• Unsaturated (kink) tails enhance fluidity

• More saturated phospholipids makes it easier for it to solidify.

• Cholesterol in eukaryotes modulates/stabilizes the fluidity of PM:• Less fluid in warmer

temp by restraining phospholipid movement

• More fluid in colder temp by preventing close packing of phospholipids.

• Membrane proteins are diverse in terms of structure, position in the membrane and function.• Cholesterol is a component of animal cell membranes.• Application: Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

Fluidity of the Plasma Membrane• Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.• The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.

• Supported by the 1970 Human-Mouse Hybrid Experiment.

• Labeled with two different fluorescent dyes.

• After a couple of hours they were evenly distributed.

Human Mouse Hybrids

“Mosaic-ness” of the Plasma MembraneMembrane proteins are diverse in terms of structure, position in the membrane and function.• Drawings of the fluid mosaic model of membrane structure can be two dimensional rather than three dimensional. Individual phospholipid molecules should be shown using the symbol of a circle with two parallel lines attached. A range of membrane proteins should be shown including glycoproteins.

• Integral proteins- generally transmembrane

• Peripheral proteins- not embedded but attached to the membrane surface.- may be attached to integral proteins or held by fibers of ECM (Extra Cellular Matrix).- on cytoplasmic side may be

http://www.susanahalpine.com/anim/Life/memb.htm

* 2 video set of videos IB Biology Topic 2.4.2 Phospholipid Properties https://www.youtube.com/watch?v=jrxnTgQDhrU IBguides's channel https://www.youtube.com/watch?v=Q_L3nylgmVY IB Biology Topic 2.4.1 Draw and Label the Plasma MembraneIBguides's channel  

Function of Membrane Proteins• Membrane proteins are diverse in terms of structure, position in the membrane and function.• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

1) Transport 2) Intercellular joining3) Enzymatic activity 4) Signal Transduction5) Cell-Cell recognition 6) Attachment to cytoskeleton

Membrane TransportEssential idea: Membranes control the composition of cells by active and passive transport.

• Selectively Permeable- Types of membrane proteins: channel, pump, carriers? - Nature of the substance: small/big, hydrophobic/hydrophilic, charged/charged?

• Non-polar Molecules- Dissolve in the membrane- Smaller move faster than larger molecules

• Polar molecules- small polar, uncharged go right between the phospholipids- Larger molecules need a transport protein- Ions also need transport help.

Transport Proteins• Essential idea: Membranes control the composition of cells by active and passive transport.

• Transport Proteins: Specific molecules or ions can pass through integral proteins• May have a tunnel ( formed

from hydrophilic amino acids)• May bind and physically move

it across membrane acting as a carrier (gated/ungated)

• Are specific for the substance they transport.

Passive Transport• Essential idea: Membranes control the composition of cells by active and passive transport.

• Passive Transport: Movement of a substance across a biological membrane.• No energy required.• Driven by the concentration

gradient (from high to low)• Rate regulated by permeability

and concentration• Example: Diffusion and Osmosis

• How do you get the most efficient diffusion?• Steep gradient• Short Distance

Diffusion vs. Osmosis• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport. • Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis. • Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions • Osmosis experiments are a useful opportunity to stress the need for accurate mass and volume measurements in scientific experiments.

• Diffusion: the tendency for molecules of any substance to spread out evenly into the available space

• Osmosis: the passive transport of water across a membrane (from high to low concentration).• Hypertonic – A solution with a greater concentration of solute.• Hypotonic – A solution with a lower concentration of solute• Isotonic – A solution with an equal amount of solute.

Diffusion vs. Osmosis

100% water

30% solute

Hypotonic

Hypertonic

• Nature of science: Experimental design—accurate quantitative measurement in osmosis experiments are essential

100% water

30% solute

Direction Osmosis Will Occur

Direction Diffusion of Solute Will Occur

Diffusion vs. Osmosis• Nature of science: Experimental design—accurate quantitative measurement in osmosis experiments are essential

Facilitated Diffusion• Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

• Facilitated Diffusion: Diffusion across a membrane with the help of transport proteins.

• Passive• Helps polar molecules and ions that are slowed down by

the membrane lipid’s nature.• They are like enzymes because

• have active sites• Max rate can be reached.• Can be inhibited.

Active Transport• Active transport is used to load organic compounds into phloem sieve tubes at the source.

• Energy is required to go against the concentration gradient (from high to low).

• Requires energy• Helps maintain steep gradients, which is necessary for the

body to work (ex. Action potentials in neurons)• Transport proteins work with ATP, which provides the

necessary energy• Examples:

- Sodium Potassium Pumps in Eukaryotes- Proton Pumps in Prokaryotes

Sodium Potassium Pump• Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axon.

• Neurons pump sodium and potassium ions across their membranes to generate a resting potential

• Three sodium are pumped out for every two potassium pumped in. Each is being pumped against the concentration gradient.

• Na/K ATPase: Main electrogenic pump (meaning it creates a voltage across the membrane) in animal cells

Proton Pump• Application: Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons

• Proton pumps: main electrogenic pump in plants, bacteria, and fungi and Chloroplasts, Mitochondria.

• By creating a voltage, it stores energy that can be used for cellular work

Bulk Transport • The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.

• Endocytosis: the transport of large molecules inside the cell by forming a vesicle from the plasma membrane

• Exocytosis: transport of large molecules out of the cell using a vesicle that has budded off the plasma membrane

Different Types of Endocytosis• The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.

Unused IB Standards• Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.• Kidney dialysis artificially mimics the function of the human kidney by using appropriate membranes and diffusion gradients.Aims:• Aim 8: Organ donation raises some interesting ethical issues, including the altruistic nature of organ donation and concerns about sale of human organs.• Aim 6: Dialysis tubing experiments can act as a model of membrane action. Experiments with potato, beetroot or single-celled algae can be used to investigate real membranes.• High concentrations of solutes in the phloem at the source lead to water uptake by osmosis